Photoperiodism is the physiological reaction of organisms to the length of day or night. Photoperiodism can also be defined as the developmental responses of organisms to the relative lengths of the light and dark periods. Here it should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods.

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Daylength, and thus knowledge of the season of the year, is vital to many animals. A number of biological and behavioural changes are dependent on this knowledge. Together with temperature changes, photoperiod provokes changes in the colour of fur and feathers, migration, entry into hibernation, sexual behaviour, and even the resizing of sexual organs.

Birds', such as the canary, singing frequency depends on the photoperiod. In the spring when the photoperiod increases (more daylight), the male canary's testes grow. As the testes grow, more androgens are secreted and song frequency increases. During autumn when the photoperiod decreases (less daylight), the male canary's testes regress and androgen levels dramatically drop resulting in decreased singing frequency. Not only is singing frequency dependent on the photoperiod but also song repertoire. The long photoperiod of spring results in a greater song repertoire. Autumn's shorter photoperiod results in a reduction in song repertoire. These behavioral photoperiod changes in male canaries are caused by changes in the song center of the brain. As the photoperiod increases so does the high vocal center (HVC) and the robust nucleus of the archistriatum (RA). When the photoperiod decreases these areas of the brain regress.[1]